Figure 5. The linear correlation between intermolecular
distance r(N…P) and the electron density (ρb) at
N…P bond critical point in R-Ph-CN…PO2F
complexes.
The NBO analysis has been conducted to characterize the orbital-orbital
interaction E (2) and the charge transfer in the
complexes studied herein, and the results are summarized in Table 3. As
expected, there is an electron charge transfer shift from the Lewis base
unit R-Ph-CN to the Lewis acid unit PO2F. As a result,
PO2F gains electron density and becomes negatively
charged, while R-Ph-CN base loses electron density and becomes
positively charged. The charge transfer (CT) is expressed as the net
natural population analysis (NPA) charge on all the atom of the electron
donor molecule (R-Ph-CN). As displayed in Figure 6(a), there is a good
linear relationship between the total charge transfer amount and the
binding distance r(N…P) with R2=0.997. The
amount of charge transfer is 0.193 e for the
Ph-CN…PO2F complex, which is larger than the
0.125 e charge transfer computed for the HCN…PO2F
complex obtained at the same B3LYP-D3/6-311+G** level. As pointed by
Grabowski [38], the
deformation energy is strongly related to the electron charge transfer,
which is often ignored in investigating the intermolecular interactions.
Here, for the dimers we studied, an excellent linear relationship is
found between the deformation energy and the charge transfer CT with
R2=0.977, as seen in Figure 6(b). For each complex,
there are two dominant orbital interactions of Lp(N) →p*(P) and Lp(N)
→σ*(P-O) in these pnicogen-bonded complexes, where Lp(N) denotes the
lone pair orbital on N atom, and p*(P) represents the empty p(π) orbital
on P atom, and σ*(P-O) denotes the P-O antibonding orbital. The
schematic diagrams of the orbital interactions in
Ph-CN…PO2F complex are displayed in Figure 7. As
indicated from the second-order perturbation energies in the last three
columns of Table 3, the contributions of the Lp(N) →p*(P) orbital
interaction ranges from 100.7 to 142.1 kcal/mol, which is far greater
than that of Lp(N) →σ*(P-O). Similar to the case of tetrel bond[49], this is a
feature of strong pnicogen-bond. It is evident from Table 3 that theE (2) energy of
R-Ph-CN…PO2F complexes follows the same order of
the strength of the interactions.